1. Field of the Invention
The present invention relates to electro-optical imagers for reading a two-dimensional matrix symbology, and more particularly, to a fixed position, high resolution imager utilizing a pin hole aperture or lens.
2. Description of Related Art
Optical imaging systems are commonly used to decipher data symbols printed on objects in order to identify the objects. A bar code symbol represents a popular form of symbology, and typically comprises a pattern of vertical bars of various widths separated by spaces of various widths. Since the bar and space elements have different light reflecting characteristics, a reader can convert the symbology into an electrical signal by analyzing the light reflected from the symbol. The electrical signal can then be decoded to provide an alphanumeric representation of the symbol which identifies the object. Bar code symbology of this nature are commonly used in various applications, such as inventory control, point of sale identification, or logistical tracking systems.
The bar code reader typically uses light that is scanned across the bar code field. The light may be laser light or light from other types of sources. Since the bar code symbology is often disposed on the object to be identified, it is desirable for the reader to be included in a hand held or portable device so that the reader can be brought to the object. Light emitting diodes (LEDs) are commonly utilized to provide the light due to their light weight and low power requirements. The operator can physically move the LED across the bar code field, such as by use of a light pen. Alternatively, a bar code reader may include movable mirrors that automatically articulate a laser light back and forth at a high rate to scan across the bar code field. The operator would normally be provided with a feedback signal, such as an audible tone, that alerts the operator as to the successful completion of a bar code reading operation.
Light weight, hand held readers are desirable for many applications, but there are several drawbacks to the use of such portable readers. Environmental variables can hinder the reading of bar code data, including: (a) the relative distance between the object to be read and the reader; (b) the orientation of the object with respect to the reader; (c) the presence of dust or dirt obscuring the bar code and/or the laser source; (d) bar code print quality inconsistencies; and (e) the intensity of ambient light altering the reflected characteristic of the light from the light source. Sophisticated readers accommodate some of these environmental variables by including lenses for focusing the light at a desired range, and apertures which adjust for the amplitude of the reflected light. While such techniques increase the ability of the reader to accurately read the bar code, the complexity of the device also increases, thus increasing the size, weight and cost of the reader.
Conventional bar code symbology has another significant drawback in that it requires a relatively large amount of space to convey a correspondingly small amount of data. Depending on the particular bar code symbology utilized, a single character of information may require numerous bars and/or spaces. To increase the amount of information presented, the width of the bar code field can be increased, however, this similarly increases the susceptibility of the reader to the environmental variables discussed above, thus increasing the difficulty of reading the data. Further, excessively wide bar codes would occupy an inordinate amount of space on the object and would be impractical for many applications. In view of these drawbacks with conventional bar code, alternative symbologies have been sought.
One such alternative symbology comprises a two-dimensional matrix that occupies a uniform amount of space with a generally rectangular or square shape. Instead of bars and spaces, round or square dots disposed at particular rows and columns of the matrix correspond to the characters being conveyed. As a result, a matrix symbology can compress significantly more data into a given amount of space than a conventional bar code. For example, a matrix symbology printed in a half inch by half inch square could convey up to two hundred characters of information. A two-dimensional matrix symbology and an associated imager is disclosed in U.S. Pat. No. 5,128,528, to Heninger.
Matrix symbology imagers convert the matrix into pixel information that is deciphered into the alphanumeric information represented by the matrix. Such imagers often utilize charge-coupled device (CCD) technology to convert optical information from the matrix into an electrical signal representation of the matrix. Unfortunately, these imagers are susceptible to the same environmental variables that afflict conventional bar code readers. Moreover, to successfully image the matrix symbology, the imager must be capable of determining the rotational orientation of the matrix. A reference symbol disposed adjacent to the matrix, such as an L or T-shaped character, can provide rotational orientation information. As with bar code readers, however, overcoming these environmental variables increases the complexity of such systems.
Accordingly, a critical need exists for a low cost, low complexity symbology imager that permits the communication of a relatively large amount of information while avoiding the complexity and drawbacks of conventional hand held bar code readers.